Glossary: Terms and components from the world of electric mobility explained in brief
The cells of lithium-ion batteries can take different forms: cylindrical cells, pouch cells or prismatic cells. Mercedes-Benz has opted to use batteries with prismatic cells. These are of the approximate shape and size of a paperback book, each in its own housing, and are sturdy and easy to work with.
Whereas a combustion engine uses a process of combustion to convert liquid or gaseous fuel into mechanical power (conventionally in the form of a stroke), an electric motor converts electrical energy into mechanical power. Energised coils generate magnetic fields with opposing forces that attract and repel to create rotational movement. Mercedes-Benz uses water-cooled asynchronous motors in the Citaro. These are robust and speed-resistant, extremely efficient to run and do not require brushes, which are subject to operational wear.
All-electric vehicles are locally emission-free, which is to say they emit no exhaust gases while in use. Depending upon how the electricity they use is produced, however, emissions may still be generated further upstream, for example in coal or gas-fired power stations. Even an all-electric city bus is therefore not classified as emission-free, but as locally emission-free.
Depth of discharge (DoD)
This describes the extent to which a battery is emptied relative to its total capacity. A very deep discharge will increase range, but has a negative impact on battery service life. In the case of the Citaro it is therefore possible to select a different depth of discharge for specific application profiles.
'High-voltage' relates to a DC voltage of more than 60 volts. High-voltage components are specifically identified as such, while high-voltage cables are recognisable by their orange sheathing. All-electric-powered commercial vehicles will commonly feature voltages of 600 volts or more. Depending on their precise role, service technicians must therefore attend mandatory familiarisation and training sessions of varying scope.
Charging standard ISO 15118
This safeguards the communication between the charging station and the on-board charge control device. It therefore provides the basis for an active and intelligent charge control system – important for the avoidance of excessive and expensive charging peaks within a fleet – and for billing systems.
The industry in Europe favours the Combo 2 connector as standard. This is suitable for high charging loads and quick charging (CCS = Combined Charging System). It allows a charging power of up to 150 kW, and currents of up to 200 A.
Battery balancing, as it is called, maintains all the cells of a battery at the same level of voltage, so ensuring its efficient operation and long service life.
Several components are contained within a single housing. The inverter, also known as an AC-DC converter, converts direct current into alternating current. It is necessary because, while the batteries supply direct current to the HV network, electric motors run on alternating current. The converter, or voltage transformer, also known as a DC-DC converter, converts incoming DC voltage into a higher or lower DC voltage.
A lithium-ion battery is a rechargeable energy storage device that converts electrical energy into chemical energy, which it then stores ready to release once again when needed in the form of electrical energy. When the battery is charged, the lithium ions move from the cathode to the anode; during discharge from the anode to the cathode. Lithium-ion batteries are renowned for their high energy density and are therefore the cutting-edge requirement for all-electric-powered vehicles.
The current collector, or pantograph, is used to pass electrical current from a charging station to a vehicle. In the case of battery-electric city buses it is used to supply a stationary vehicle during so-called opportunity charging, and occasionally for charging at the depot. The device may be fitted as an integral feature on the roof of the bus or as a fixed installation at charging stations. In either case the current collector is extended at the beginning of the charging process to connect the vehicle with the charging station.
The peak output of an electric motor is comparable with the figure given for the rated output of a combustion engine and corresponds to the maximum possible output that is briefly achievable. Another quoted figure is that for continuous output, representing the maximum output that can be called upon for an extended period.
A display in the instrument panel that replaces the rev counter commonly found in vehicles with a combustion engine. The power meter shows the level of output currently being drawn by the drive systems. During braking or coasting, it shows the level of recuperation, i.e. of braking energy being recovered.
Wheel hub motor
Unlike a combustion engine, the position of the electric motor within the vehicle is largely immaterial. Various configurations are possible for city buses. Mercedes-Benz has opted for a compact form of installation - the tried and tested ZF AVE 130 drive axle with electric motors mounted at the wheel hubs, as previously deployed in other Citaro models. This electric portal axle features a water-cooled asynchronous motor at each wheel, each delivering 125 kW, in other words together 250 kW. From standstill, the motors therefore deliver torque of 2 x 485 newton metres, which the gearing ratio turns into a drive torque of approx. 2 x 11 000 newton metres. The space freed up by the absence of the combustion engine and transmission on the left-hand side at the rear is used by Mercedes-Benz to accommodate battery modules.
Recuperation is also known as braking energy recovery or regenerative braking and is a process whereby kinetic energy is converted into electrical energy. The wheel hub-mounted electric motors in this case function as alternators, leading to the creation of braking torque. The electrical energy generated in this way is stored in the batteries, thereby becoming available again for use by the vehicle's drive system.
A reproducible on-road test cycle used to calculate comparable fuel consumption figures for city buses. SORT (Standardised On-Road Test cycles) is based on a series of agreements between trade associations, transport operators and leading European bus manufacturers. SORT replicates a range of challenging urban traffic scenarios (SORT1, SORT2, SORT3). SORT figures may, for instance, be used as a criterion in calls for tender.
A heat pump exploits the physical effects of the transition of liquids to a gaseous phase, or vice versa. It is a process that is already familiar from modern stationary heating systems in buildings. In all-electric vehicles, heat pumps are used as an alternative source of energy-efficient heating or cooling for the vehicle.